1. Field of the Invention
This invention relates to molten carbonate fuel cells and their component structure and assembly. The invention simplifies the assembly of components and start-up procedure for molten carbonate fuel cells and stacks produced by the tape casting method by reducing the number of components required for assembly and greatly reducing or eliminating the requirement of loading active electrolyte into the electrolyte matrix.
2. Description of Related Art
Presently, fuel cells produced by the tape casting method are assembled with an anode, a cathode, two or more layers of electrolyte matrix tapes and a source(s) of active electrolyte for the matrix and electrodes. The tapes contain organic binders which are burned off as the assembled cell is heated to its operating temperature of typically about 600.degree. to 750.degree. C. After the binders are removed, the alkali metal carbonates electrolyte is melted and impregnates the cell components. Another method of fuel cell electrolyte production is hot pressing of mixed powders of electrolyte matrix and active electrolyte. However, high pressures are necessary and commercially practical sizes of electrolytes for molten carbonate fuel cells cannot be produced by hot pressing. To achieve commercial practicability of molten carbonate fuel cells, the electrolyte must be very thin, on the order of 10 to 70 mils, to provide sufficiently low IR drop, be stable under the broad range of thermal cycle necessary upon cell shut-down and start-up, be capable of maintaining gas integrity under these different conditions, and be practical to produce. In addition, the electrolyte structure comprising carbonate electrolyte and LiAlO.sub.2 support particles should have as low an empty porosity as possible.
U.S. Pat. No. 4,555,453 teaches electrophoretic deposition of electrolyte matrix, with or without active electrolyte material, from a dielectric vehicle upon the surface of a fuel cell electrode. The '453 patent teaches the electrolyte matrix may be electrophoretically applied to one electrode and active electrolyte impregnated into the matrix or the active electrolyte may be impregnated into the electrolyte matrix powder and both simultaneously electrophoretically deposited onto the electrode. The '453 patent teaches thicknesses of less than 500 microns are possible since the electrolyte is mechanically supported on the fuel cell electrode. U.S. Pat. No. 4,329,403 teaches deposition of electrolyte material on electrodes wherein the electrolyte matrix has a varying composition, the sides towards the electrodes having coefficients of expansion nearer to that of the adjacent electrode.
U.S. Pat. No. 4,538,348 teaches electrolyte matrix tapes for molten carbonate fuel cells which tapes are produced by casting a mixture of inert particles less than 1 micron in size to produce fine pores, corrosion resistant ceramic particles having an average size greater than 25 microns for crack attenuation, and a plastic binder which is volatilized from the cast tape. The fuel cell is then assembled and active electrolyte contained in a prefilled porous anode is wicked into the electrolyte matrix upon heating to fuel cell operating temperature.
U.S. Pat. No. 4,216,278 teaches fabrication of fuel cell electrolyte matrix blanks by molding, pressing or tape casting followed by impregnating the electrolyte matrix blank through contact of molten active electrolyte with one surface of the blank under furnace conditions.
U.S. Pat. No. 4,526,812 teaches an electrolyte carbonates coated ceramic powder for preparing electrolyte porous tapes for use in molten carbonate fuel cells. The ceramic particles are entirely coated with 5 to 30 volume percent electrolyte carbonates which does not significantly change the size of the ceramic particle. The '812 patent teaches the necessity of adding electrolyte carbonates to the cell to fill the porosity. Molten carbonates fuel cells tested according to the '812 patent showed peak performance of about 0.725 to 0.775 volts at 160 ma/cm.sup.2 and 650.degree. C.
U.S. Pat. No. 4,009,321 teaches electrolytes for molten carbonate fuel cells wherein the inert electrolyte carrier material is substantially pure alkali metal aluminate; U.S. Pat. No. 4,079,171 teaches lithium aluminate support matrix having specific crystalline structure for molten carbonate fuel cell electrolytes; U.S. Pat. No. 4,115,632 teaches a method of preparation of fuel cell electrolytes of rod-shaped lithium aluminate particles; and U.S. Pat. No. 4,663,250 teaches addition of an electrode dissolution reduction agent to an electrolyte for use in a molten carbonate fuel cell.